I think the only possible solution is to use map entity for each wall or divide each wall to several map entities and then you will be able to use "you" pointer to set the transparent flags and play with alpha .. which is bad idea to change your entire level design just to make that possible
I think the block once combild cant change their flags

how about to change your mind to make the camera come closser to the player insted of making walls transparent .. and the player turn to invesible when it stuck between the wall and the player (i think this already been done by other members) .. or to move the camera in front the player when there is no space .. or to move it above the player .. or combine the last two methods like what max payne camera do .. I know that is not what you want .. I previously want to do the same as what are you wishing to do .. but its obvious its not possible without map entities blocks

Regards